1. Field of the Invention
The present invention relates to a monolithic piezoelectric transformer for use in an inverter for the backlight of a liquid crystal display, an inverter for starting a fluorescent tube, or the like, and to a method for manufacturing the same.
2. Description of the Related Art
A piezoelectric transformer has been proposed as a miniature power supply for generating a high voltage in order to reduce the size of electronic apparatuses driven by the high voltage, such as inverters of transistor TV sets and for starting fluorescent tubes.
Monolithic piezoelectric transformers generally have a rectangular body. The body includes a drive portion and a power generation portion adjoining the drive portion in the longitudinal direction.
The power generation portion has a piezoelectric member polarized in the longitudinal direction of the body and an output electrode formed on an end surface in the longitudinal direction of the body. The drive portion includes internal electrodes and piezoelectric ceramic layers that are alternately layered in the thickness direction, and a pair of external input electrodes. The piezoelectric ceramic layers are alternately connected to one of the external input electrodes or the other.
In a process of preparing such a drive portion, piezoelectric ceramic green sheets onto which an internal electrode paste has been applied in an electrode shape are layered and contact-bonded to one another in the thickness direction to form a rectangular monolithic piezoelectric composite. The monolithic composite is fired, and thus, the drive portion is obtained.
A known internal electrode paste mainly contains a silver-palladium conductive powder, and further contains a gold resinate as an organic metal material for controlling the sintering of the silver.
The thickness of the piezoelectric ceramic layers is often increased in order to adjust the step-up ratio of a monolithic piezoelectric transformer. In this instance, however, the diffusion of the silver in the composition progresses to seriously degrade the continuity of the electrodes if the internal electrodes are formed of such a conventional composition. Consequently, the strength of the ceramic layers is negatively affected and the characteristic values are varied.
Accordingly, an object of the present invention is to provide a monolithic piezoelectric transformer using an internal electrode material capable of maintaining the continuity of the electrodes after firing even if the thickness of the piezoelectric ceramic layers is increased.
To this end, there is provided a monolithic piezoelectric transformer according to one aspect of the present invention, having a plurality of piezoelectric ceramic layers that are integrally sintered. The monolithic piezoelectric transformer includes a drive portion and a power generation portion. The driver portion includes the plurality of piezoelectric ceramic layers polarized in the layered direction, a plurality of internal electrodes alternately layered with the piezoelectric layers, and a pair of external input electrodes connected to the internal electrodes. The power generation portion includes a piezoelectric ceramic member polarized in the direction perpendicular to the layered direction and an output electrode. The internal electrodes comprise a primary metal comprising at least one of silver and palladium and a secondary metal comprising at least one of rhodium and iridium.
Since the internal electrodes contain the secondary metal, the primary metal is prevented from diffusing into the piezoelectric ceramic layers during sintering. Consequently, the continuity of the internal electrodes is maintained. Also, a decrease of ceramic layer strength resulting from the diffusion of the primary metal is prevented. Thus, the variations in the properties of the electrodes can be reduced.
Preferably, the content of the secondary metal is in the range of about 0.01 to 5 parts by weight relative to 100 parts by weight of the primary metal. If the secondary metal content is less than about 0.01 part by weight, the primary metal cannot sufficiently be prevented from diffusing into the piezoelectric ceramic layers during sintering. If the secondary metal content is more than about 5 parts by weight, the diffusion is reduced to some extent but the characteristics are not satisfactorily improved and costs are increased.
Preferably, the secondary metal is in a metal organic form before sintering. By using a metal organic for the secondary metal, the primary metal is further prevented from diffusing into the piezoelectric ceramic layers during sintering. Thus, the variation of properties can further be reduced.
According to another aspect of the present invention, a method for manufacturing a monolithic piezoelectric transformer is provided. The method includes the step of applying an internal electrode paste containing a primary metal comprising at least one of silver and palladium and a secondary metal comprising at least one of rhodium and iridium onto green sheets containing a piezoelectric ceramic material to form an internal electrode paste layer on each green sheet. The method also includes the steps of layering the green sheets with the internal electrode paste layer on one another to prepare a piezoelectric ceramic composite; and firing the piezoelectric ceramic composite. In another step, the fired piezoelectric ceramic composite is polarized in the layered direction thereof to prepare a drive portion, and the fired piezoelectric ceramic composite is polarized in the direction perpendicular to the layered direction to prepare a power generation portion. In still another step, a pair of external input electrodes connected to the internal electrode paste layers is formed on the drive portion, and an output electrode is formed on the power generation portion.
According to the method of the present invention, the foregoing monolithic piezoelectric transformer can be manufactured with a good reproducibility.
Preferably, the content of the secondary metal in the internal electrode paste is in the range of about 0.01 to 5 parts by weight relative to 100 parts by weight of the primary metal. Preferably, the secondary metal is in metal organic form.